1. Field of the Invention
The present invention relates to a wavelength-multiplexing optical transmission system and a wavelength-multiplexing optical transmission method.
2. Description of the Related Art
In the field of optical communication networks, a wavelength division multiplexing (WDM) system has been mainstreamed nowadays. In the WDM system, a plurality of optical signals with different wavelengths is multiplexed into a single optical fiber cable to increase a transmission capacity of the optical fiber cable. To establish the WDM system, a transponder is required with a capability of converting an optical signal with a broadband wavelength into an optical signal with a narrowband wavelength. With the increase of the number of multiwavelength transmissions in recent years, the transponder needs to respond to multiple wavelengths. Therefore, a tunable transponder capable of tuning the wavelength is generally used in the WDM system.
To cause the tunable transponder to output an optical signal with a specified optical wavelength, it is necessary to set the optical wavelength on the tunable transponder. In a conventional technology, as shown in FIG. 18, the wavelength is manually set by an operator. Specifically, the operator first confirms information on a physical connection between a transponder and a wavelength multiplexing unit or a wavelength demultiplexing unit, which are included in a wavelength-multiplexing optical transmission apparatus. Particularly, the operator confirms information on connections between the transponder and each port of the wavelength multiplexing unit or each port of the wavelength demultiplexing unit, and then manually sets the wavelength on the wavelength-multiplexing optical transmission apparatus via a control terminal. The operator's instruction for setting the wavelength is transmitted from the control terminal to a control unit of the wavelength-multiplexing optical transmission apparatus. The ports of the wavelength multiplexing unit and the wavelength demultiplexing unit are configured to respond to specified optical wavelengths, respectively. Therefore, information on the optical wavelengths corresponding to the ports is also manually set via the control terminal at the same time (see FIG. 18 (1)). Then, the control unit of the wavelength-multiplexing optical transmission apparatus informs a control unit of the transponder about the information on the optical wavelengths (see FIG. 18 (2)). Upon being informed the information, the control unit of the transponder sets the optical wavelengths on the transponder based on the information informed from the control unit of the wavelength-multiplexing optical transmission apparatus (see FIG. 18 (3)).
However, with the above method, a chance of causing an error is high because the setting process is performed manually. In addition, a workload on the operator disadvantageously increases because the operator needs to perform manually all the processes of confirming the information and setting the optical wavelength.
To solve the problems, Japanese Patent Application Laid-open No. 2004-274113 discloses a method of setting an optical wavelength on a transponder based on an intensity of a light output from the transponder. Furthermore, Japanese Patent Application Laid-open No. 2005-277686 discloses a method of setting an optical wavelength on a transponder by detecting a light output from the transponder.
The method disclosed in Japanese Patent Application Laid-open No. 2004-274113 is explained in detail below with reference to FIG. 19. The transponder sequentially outputs an optical signal to a wavelength-multiplexing optical transmission apparatus by changing an optical wavelength of the optical signal (see FIG. 19 (1)). When receiving the optical signal from the transponder, the wavelength-multiplexing optical transmission apparatus detects light intensities of optical signals before and after a band-pass filter, and calculates a difference value between detected light intensities (see FIG. 19 (2)). The above processes (1) and (2) are repeatedly performed by the transponder and the wavelength-multiplexing optical transmission apparatus, respectively. When the smallest difference value is obtained, the wavelength-multiplexing optical transmission apparatus determines that an optical signal with an optical wavelength appropriate to the band-pass filter is output from the transponder, and informs the transponder about the optical wavelength (see FIG. 19 (3)).
The method disclosed in Japanese Patent Application Laid-open No. 2005-277686 is explained in detail below with reference to FIG. 20. The transponder sequentially outputs an optical signal to a wavelength-multiplexing optical transmission apparatus by changing an optical wavelength of the optical signal (see FIG. 20 (1)). Upon receiving the optical signal via an optical multiplexer included in the wavelength-multiplexing optical transmission apparatus, the optical multiplexer outputs the optical signal to a photodiode included in the wavelength-multiplexing optical transmission apparatus. Then, the photodiode outputs a detection signal (an electrical current) corresponding to the received optical signal to a light-emitting diode included in the wavelength-multiplexing optical transmission apparatus (see FIG. 20 (2)). Upon receiving the detection signal, the light-emitting diode emits a light corresponding to the received detection signal. The emitted light becomes a feedback light to the transponder (see FIG. 20 (3)). The feedback light is detected by a photodiode included in the transponder. Then, the photodiode informs a wavelength control unit included in the transponder about a detection signal corresponding to the feedback light (see FIG. 20 (4)). Each port of the optical multiplexer is configured to pass only an optical signal with a specified wavelength therethrough. Therefore, the time in which the photodiode included in the wavelength-multiplexing optical transmission apparatus receives a light indicates the time in which the transponder outputs an optical signal with a wavelength appropriate to a port of the optical multiplexer. In other words, the time in which the transponder detects a feedback light indicates the time in which the transponder outputs an optical signal with a wavelength appropriate to a port of the optical multiplexer.
In the above conventional technologies, however, it is not possible to set an optical wavelength stably. For example, if there is an abnormality in the wavelength-multiplexing optical transmission apparatus, it may not be possible to detect a light intensity or a light itself precisely.
In the method disclosed in Japanese Patent Application Laid-open No. 2004-274113, an optical wavelength is set on the transponder based on an intensity of a light output from the transponder. Therefore, in case the wavelength-multiplexing optical transmission apparatus has a trouble (for example, because of a defect in the band-pass filter), an intensity of a light cannot be detected precisely, and thus an optical wavelength cannot be set on the transponder stably. Similarly, in the method disclosed in Japanese Patent Application Laid-open No. 2005-277686, an optical wavelength is set on the transponder upon detection of a light output from the transponder. Therefore, in case the wavelength-multiplexing optical transmission apparatus has a trouble (for example, because of a defect in a port of the optical multiplexer), a light itself cannot be detected, and thus an optical wavelength cannot be set on the transponder stably.